Gas turbine engines are known and, typically, include a fan delivering air into a bypass duct as propulsion air. The fan also delivers air into a core engine where it passes to a compressor. The air is compressed in the compressor and delivered downstream into a combustion section where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors driving them to rotate.
Historically, the fan rotor and a fan drive turbine rotor have been driven at the same speed. This placed a restriction on the desirable speed of both the fan and the fan drive turbine.
More recently, it has been proposed to provide a gear reduction between the fan drive turbine and the fan rotor.
The gear reduction is a source of increased heat loss. As an example, a geared turbofan engine creates about twice as much heat loss as a non-geared turbofan engine. In addition, the weight of the engine increases due to the weight of the gear reduction.
It has typically been the case that a designer of a gas turbine engine sizes an oil tank such that the oil can sit in the oil tank long enough to de-aerate. On a normal turbofan engine, this had been approximately at least ten seconds.